The present invention relates to surface-coated SiC whiskers, processes for preparing the same, a ceramic reinforced with the same, and a process for preparing said reinforced ceramic.
More specifically, the present invention relates to SiC whiskers having a surface coated with a thin carbonaceous layer or an Si.sub.3 N.sub.4 layer, a process for preparing the same, a ceramic reinforced with the same, and a process for preparing said reinforced ceramic.
Ceramics are excellent in thermal resistance, high-temperature strength, and chemical stability, and are used as various structural members including an engine part, which are used at high temperatures.
In general, ceramic materials are so unsatisfactory in fracture toughness that concentration of stress is liable to occur by a minute crack or an internal defect, leading to fracture. Therefore, a method of improving the fracture toughness has recently been widely investigated and various attempts to reinforce a ceramic with SiC whiskers to improve the toughness of the ceramic have been proposed [see, for example, Am. Ceram. Soc. Bull., 64[2], 298 to 304 (1985)]. In these attempts, the fracture toughness of a ceramic is improved by dispersing highly elastic whiskers in the ceramic according so as to terminate or control the growth of a crack in the ceramic, to deviate the direction of progress of a crack to thereby relax concentration of stress, or to absorb crack developing energy with a whisker present at the tip of a crack based on the pull-out of the whisker, or prevention of crack development with the whisker. However, when SiC whiskers are simply directly dispersed in a ceramic, the state of the interfacial bond between the SiC whiskers and the ceramic gravely influences the improvement in the fracture toughness of the ceramic. When the interfacial bond is strong, the effect of improving the fracture toughness is insufficiently exhibited. On the other hand, when the interfacial bond is weak, the SiC whiskers disadvantageously provide starting points of occurrence of cracks.
In view of the above, there have been proposed methods in which the surfaces of SiC whiskers are coated with other material in order to control the interfacial bond between the SiC whiskers and a ceramic and the surface-coated SiC whiskers are dispersed in the ceramic.
For example, there is known a method of preparing a composite in which the surfaces of SiC whiskers are coated with carbon by the chemical vaper deposition method and the obtained carbon-coated SiC whiskers are dispersed in A1.sub.2 O.sub.3 [see 25th Yogyo Kiso Toronkai Koen Yoshishu, p. 41 (1987)]. In this method, however, only SiC whiskers having a carbon coating layer having a thickness which is large (about 450 .ANG.) and is not uniform are obtained so that the strength and fracture toughness of the ceramic are rather lowered.
As described above, although the improvement in the fracture toughness of a ceramic which serves as a high-temperature anti-corrosive structural member is a very important problem, no effective improving means have been found.